1. Field of the Invention
The present invention relates to sheet-through document reading apparatuses, particularly to a document reading apparatus operable to purposely form a slack in a document between a resist convey unit and a read convey unit.
2. Description of Related Art
In conventional document reading apparatuses, each document placed on a document tray is conveyed to an image reading portion after passing a separation roller, a resist roller, and a read roller, as described in Japanese Patent Laid-Open Publication No. 2001-146337. Here, document conveying velocity v1 at the resist roller is set slightly higher than conveying velocity v2 at the read roller, so that a slack is formed in the document in a U-turn path between the resist roller and the read roller. As a result, a velocity variation at the time of the trailing edge of the document exiting the nip of the separation roller is accommodated by the slack formed in the document.
However, such conventional document reading apparatuses have a problem of not being able to set an appropriate amount of the slack in accordance with the length of a document.
Furthermore, the conventional document reading apparatuses have another problem of not being able to set an appropriate amount of the slack in accordance with the grammage of a document. Specifically, if a thick, i.e., high grammage, document is conveyed, the resist roller readily causes slippage, so that the actual conveying velocity of the resist roller becomes unstable. As a result, an amount of the slack to be formed in the document in the U-turn path fluctuates in accordance with the grammage.